Adsorption drying is a chemical process in which water vapor is bound to adsorption material that can either be a solid or liquid.
The general working principle of adsorption dryers is simple: moist air flows over hygroscopic material — typically silica gel, molecular sieves or activated alumina — and is then dried. The exchange of water vapor from the moist compressed air into the hygroscopic material or desiccant causes the desiccant to gradually become saturated with adsorbed water.
Therefore, adsorption dryers are typically built with two drying vessels to regularly regenerate the desiccant so it regains its drying capacity. The first tower dries the incoming compressed air while the second tower is being regenerated. Each tower switches tasks when the other tower is completely regenerated. These dryers are suitable for providing very, dry air for more critical applications since a typical pressure dew point of -40°F can be achieved.
There are four different ways to regenerate the desiccant material, and the method used determines the type of adsorption dryer.
Here are the four types of adsorption dryers:
- Heatless-type dryers
These dryers are best suited for smaller air flow rates, high inlet temperatures and very low dew points. The regeneration process takes place with the help of expanded compressed air, or purge air, and requires approximately 15 to 20 percent of the dryer’s nominal capacity to regenerate the media bed.
- Heated purge regenerated dryers
These dryers heat up to the expanded purge air by means of an electric air heater. They use 25 percent less energy than heatless-type dryers while limiting the required purge flow to around 10 percent.
- Blower regenerated dryers
Ambient air is blown over an electric heater and brought into contact with the wet desiccant to regenerate it. With this type of dryer, compressed air isn’t used to regenerate the desiccant material, which helps lower the energy consumption to 40 percent less than heatless-type dryers.
- Heat of compression dryers
In heat of compression dryers, the desiccant is regenerated by using the available heat of the compressor. Instead of evacuating the compressed air heat in an aftercooler, the hot air is used to regenerate the desiccant. This type of dryer provides a typical pressure dew point of -4°F without adding any energy. A lower pressure dew point can also be achieved by adding extra heaters.
Separation and drainage of the condensation water must always be arranged prior to adsorption drying. If the compressed air was produced using an oil-lubricated compressor, an oil coalescing filter must also be fitted upstream of the drying equipment. For more information on proper equipment to accompany your adsorption dryer, reach out to an Atlas Copco expert.